The present invention relates generally to the field of autotransformers and more particularly to autotransformers employing printed wire board windings.
The demand for a low weight, low-cost and high-power-density transformer has pushed the transformer made through some traditional manufacturing methods to its limit. Generally, as current is run through a transformer, the wire resistance generates energy loss as heat.
FIG. 1 is a schematic of an exemplary three phase, 18-pulse autotransformer circuit 100 according to the prior art. Such an electrical circuit is one design used in aerospace applications to meet certain input harmonic distortion requirements. The autotransformer has three phases: Phase 1; (150), Phase (2); 160, Phase 3; (140). Phase 1 (150) includes an input 110 (P1), three outputs 130, (S3, S7, S5), and six windings 120 (A-F). Phase 2 (160) includes an input 110 (P2), three outputs 130, (S1, S6, S8), and six windings 120 (A-F). Phase 3 (140) includes an input 110 (P3), three outputs 130, (S2, S4, S9), and six windings 120 (A-F). For illustrative purposes, the windings of respective phases may be considered interchangeable, in other words Phase 1 winding 120 F may be equivalent in gauge and turns as Phase 2 winding 120 F and Phase 3 winding 120 F.
In some traditional transformers, the windings are individually insulated magnetic wires wrapped in direct contact around a metallic core creating an upper half and a lower half of windings. It is known in the art to couple a heat sink plate to a transformer in an effort to draw heat away from the windings. In general, the bottom surface of a transformer winding is used for interface to the heat sink plate to remove the heat; however, an insufficient amount of winding bottom surface is generally flat enough and available for efficient thermal conduction.
It is also known in the art to further increase the power density in a transformer by using copper strips to draw the heat out parallel along a surface of the transformer core. Referring to FIGS. 2-3 an example prior art three phase 18-pulse autotransformer 101 is depicted in accordance with the schematic of FIG. 1. A transformer core 170 is inserted within three phases, Phase 3 (140), Phase 1 (150), and Phase 2(160). Each phase includes respective windings 120 with wire connections protruding from the windings serving as the inputs 110 (P3, P1, P2) Phase 3 also includes three outputs 130 (S2, S4, S9) that are similarly protruding wire connections as the inputs 110. Phase 1 similarly includes three outputs 130 (S3, S7, S5) and Phase 2 also includes three outputs 130 (S2, S4, S9). The flat surface area at the bottom of each phase winding may be about 40% of the total bottom surface area. One end of copper strips 180 are inserted under the core and in between winding layers. Heat is drawn out along a heat path HP along each strip where the other copper strip end may be in contact with a heat sink (not shown).
Referring specifically to FIG. 3, a cross-sectional side view of an exemplary phase in accordance with a transformer of the prior art shown in FIG. 2 is depicted. The windings 120 are designated A-F types in correspondence with similarly labeled windings in FIG. 1. The windings 120 are insulated from one another by insulation 185 typically 0.2 mm thick. Windings 120 may generally escalate in gauge thickness the closer the winding is to the core where winding types E are the outermost windings and winding types A are the innermost. Thus, a hot spot HS may build up in a localized area in the innermost windings as heat dissipation is hindered by the insulation wires and an obstructed path to the heat sink. This approach can increase the weight and price and also may limit heat sink performance by creating a long heat path. A hot spot can build up in the winding half that is not in contact with the copper strip and heat from that spot may need to travel through wire insulation, other winding layers and sometimes the core and other winding half until it reaches the copper strip. The copper strips can also add more space at the bottom of the transformer making for a non-planar surface which can make cooling of the transformer core through a supporting bracket less effective.
It is further known in the art to manufacture transformers employing printed wire boards that include trace windings. One example uses spiral windings on stacked and staggered individual printed boards to form primary and secondary windings and electrically connecting the windings to the main circuit board by internal vias as seen in U.S. Pat. No. 6,914,508 to Ferencz et al. Such designs do not address the heat path built up during heat generation. Additionally, they suffer from needing to stack together non-uniform sized printed boards and do not address forming electrical connections between the boards.
It is also known in the art to use printed wire boards to form a transformer connected together by using variable position vias and a pin and jumper system as shown in U.S. Pat. No. 6,628,531 to Dadashar. These kinds of printed wire board stacks suffer from not addressing heat path issues and also from requiring offset stacking in the interconnection of boards.
As can be seen, there is a need for an autotransformer using a printed wire board design that creates an improved heat path for withdrawal of heat from trace windings. Furthermore, it can be seen that there is a need for an improved interconnection of printed wire boards.